MrMom: With regard to RYVNX, if you buy if from a broker other than Rydex directly, the cutoff time is 5 or 10 minutes before the market closes. Check with Ameritrade on that.
It is bought and sold just as if it were a mutual fund and is not suitable for day trades or, in my opinion, swing trades of less than 5 days duration. It is an easy way to lose a lot of money quickly and should be used for holding over an intermediate term period (e.g., ideally, greater than 1 month). It is for making a big bet on the market --- that we are near a top, with very limited upside and significant downside (or vice versa).
There are no excessive trading rules ( or, there shouldn't be) -- if Ameritrade hassles you about that with Rydex or ProFunds, switch brokers or open an account with Rydex or ProFunds directly. The main problem is "slippage" --- you will only get the closing buy or sell prices.
Hope this helps.